Ancient Persians flipped a series of drawings on a thin flexible material, with the use of their hands, to create apparent motion in the figures and animals depicted on the material. Each picture was drawn in a slightly different position in stages of motion. It is a very old discovery, whose surprising result is still employed today in children's toys. The capacity to blend changing, different images smoothly into flowing motion has other modern uses as well. Motion pictures and television are examples of this form of perception, a phenomenon which is now known as the "persistence of vision", defined as the capacity of mechanisms associated with the eye and sight to retain the image of something seen after the removal of the stimulus that produced it.
This property of the eye interferes with the intended function of devices such as movie projectors and stroboscopes, which block light and rapidly extinguish light, respectively. In order not to allow awareness of change from frame to frame as a movie film is being moved through a projector, there is typically a shutter mechanism which interrupts light emitted by the projector's lamp between frame changes. In a typical projector, the shutter blocks the light approximately one third of the time. Yet, without film in the projector, the area of light projected on a surface through the lens appears unchanging and nearly of the same intensity.
In contrast, stroboscopes, such as those disclosed in U.S. Pat. No. 2,936,387 to Steele et al. and U.S. Pat. No. 3,633,161 to Price, produce a very brief interval of light which has the ability to produce the appearance of interrupted movement, or unconnected motion, to an observer. The period of darkness between bursts of light, and the length of each period of darkness relative to the duration of the burst of light, is intended to prevent the appearance of smooth motion by overcoming the persistence of vision effect. A commonly known application where this effect is utilized is when a strobe light is used to make dancers on a dance floor appear to move with a jerking and unconnected motion. In other applications, stroboscopes are employed to seemingly freeze motion. For example, machinery which have rotating or reciprocating parts can be illuminated with a short lived burst of light at the same frequency at which the part rotates or reciprocates into repetitive positions. When the frequency of light pulses matches the part's rate of rotation or reciprocation, the effect of ambient light is overcome and the part appears motionless, enabling its speed to be known. In effect, stroboscopes used for such purposes operate in a manner which overcomes the persistence of vision effect.
U.S. Pat. No. 5,010,410 to Garriss suggests that the capabilities of a stroboscope be utilized with video camera technology to produce an intermittent light whose frequency and duration coincide with the frequency with which a film frame is exposed when analyzing an object that is being inspected with the camera. In effect, the object is illuminated by sufficient light during the exposure of a frame such that the light intensity at certain locations on the object can be used to determine the shape of the object. The light source, in this case a number of light emitting diodes, is energized only while the camera is acquiring an image, and is thereafter off to minimize power consumption, heat and noise.
While the apparatus taught by Garriss achieves an effect in which, while viewing a film, a continuous image could be perceived by a viewer, the actual recording process operates in a manner which is contrary to the persistence of vision effect. Specifically, the video camera operates in a manner which forms one or more distinct individual images during filming, with the light source being on for a duration sufficient only for the camera to acquire an image. Consequently, each light cycle is dedicated to exposing the film, and does not serve to produce sufficient light at a frequency and for a duration which are necessary to produce the persistence of vision effect.
Each of the above examples are cited to distinguish devices which are intended to operate in a manner which serves to defeat the persistence of vision effect. However, modern examples do exist in which the persistence of vision effect is advantageously used to achieve an apparently continuous light at a substantially uniform intensity. For example, a television set scans a new picture approximately 60 times per second, with a small point of illumination. The picture is never there all at once, as the viewer perceives it to be, illustrating that constant light is not necessary to enable normal vision. However, the prior art is lacking in any suggestion that the persistence of vision effect can be utilized to produce adequate lighting for normal vision. Because a continuous light source is not necessary to produce the appearance of continuous light, the persistence of vision effect could be advantageously exploited to provide lighting which, although appearing to be continuous and conventional, is in fact intermittent, such that energy could be conserved. The potential for such an improvement would be highly desirable, particularly in view of the considerable activity in the area of light generation, and efforts to improve the life and efficiency of light sources.
Thus, it would be desirable to provide a device by which the persistence of vision effect is employed to produce artificial lighting. Such a device would operate by emitting light which is interrupted in some manner so as to allow the persistence of vision effect to provide an observer with the sensation that light is continuously and uniformly being emitted by the light source.